1. Field of the Invention
The present invention relates generally to the art of food warmers and more particularly to food warmers which are used to maintain wrapped or unwrapped food products at a desired temperature during the holding period between food preparation and delivery to the purchaser. In its preferred embodiment, the present invention relates to a self-regulating warmer plate in conjunction with infra-red heaters spaced apart and above the plate for use in maintaining food products at desirable temperatures in restaurants and fast food establishments.
2. Description of the Prior Art
It has been known for many years that food products must be maintained at elevated temperatures between the time they are prepared and the time they are served if there is any appreciable amount of time between the two. It is necessary to warm such foods if a time delay exists for two main reasons: the food must be warm to please the customer and the food must be kept warm to prevent bacteriological contamination. Using fast food restaurants as an example, all states have health codes requiring that food products, such as hamburgers, be maintained at a certain temperature, typically in the range of 140.degree.-150.degree. F.
Numerous devices have been introduced to the marketplace to accomplish the desired heating. One known system includes a planar sheet onto which the wrapped or unwarpped food products are placed and an infra-red heating system is placed above the sheet with rays from its heating bulbs directed downwardly toward the plate. By way of example, many fast food restaurants use such systems for serving a variety of sandwiches, and typically the devices used in these outlets are inclined and include divider rods, so that the sandwiches will slide in a confined path from an upper side (usually in the kitchen area) to a lower front edge where they are readily accessible to the employees serving the customers. Many shapes, sizes and heating arrangements are known for such devices, but to understand the present invention it is necessary to understand the types of problems resulting from use of these previously employed heaters.
First, the food heaters now in use are constantly being subjected to changes in ambient temperature conditions. For example, any heater which is located near an entrance door or an opening for drive-thru service may be exposed to chilling gusts of cold air. Temperatures surrounding the product holding area can also change significantly depending on the particular cooking (frying, baking, etc.) operations taking place in the kitchen area, as well as seasonal temperature changes where air conditioning or heating air flow may cause an effect.
Second, the uniformity of heating provided by known devices is significantly affected by the number and area distribution of the food objects placed on the sheet and beneath the infra-red heaters. Again using the fast food chain outlet as an example, it will be readily appreciated that a sandwich heater may include a number of rows of paper wrapped products and a number of products contained in sheet foam containers. The amount of surface area of the sheet exposed to the infra-red heaters and the location of such exposure will be constantly changing as the various products are ordered and served customers, and as the inventory of the various products is replenished from time to time by the store's chefs. Accordingly, in some areas the rays from the upper heater will be directed onto the sheet and heat the sheet from above, while in other areas, the food product will intercept and be warmed by the infra-red rays, while the sheet therebeneath will be isolated from exposure to the heating rays.
It has also been noted in these prior art devices that a considerable amount of heat is lost from the edges of the sheet, for obvious reasons, and all the aforementioned drawbacks have been addressed in part by newer systems which include a heating element in the sheet itself. In the newer systems known to the present inventor, a resistance heating wire, encased by a surrounding insulator and placed in a tape like strip of heat conductive material, such as aluminum foil, has been used. The strip is placed around the periphery of the sheet in an attempt to prevent edge heat loss. A sensor, such as a thermostat bulb is located on the lower surface of the sheet to sense the temperature of the sheet and to control when current is supplied to the wire to cause uniform heating of the strip.
Several drawbacks also exist with regard to the latter system. First, the entire sheet is heated when the thermostat senses a need for heat at the particular location of the bulb. The problem here, is that only that one area may need heat, and overheating of the other areas on the sheet will occur. Second, the application of heat around the entire perimeter of the sheet, when only local heating is required, results in wasteful energy consumption.
While these prior systems have been explained to this point with reference to fast food service slides for sandwiches, it will be readily apparent to those skilled in the art that similar problems exist in all areas of the food service industry. Just for a few examples to illustrate this point, consider a warming tray for rolls or baked potatoes at restaurants, or the holding tray for french fries in any type of food serving situation. It is readily seen then that the problem is one which exists and for a variety of wrapped and unwrapped foods.
Regulating heaters are known, and by way of example, Raychem Corporation of Menlo Park, Calif. has introduced in recent years a heater element that has provide desired results in accordance with the teachings of this invention. These heaters and are described in a 1985 technical brochure entitled "Raychem Self-Regulating Heaters". The technology involved with such heaters involves providing a pair of parallel electrodes (wires) in an insulated cable. A blend of synthetic resin and current conducting carbon particles is placed between the electrodes and is manufactured in such a way that at low temperatures current flow is permitted between the electrodes. As temperatures increase, the electrical resistance of the carbon-resin system increases thus reducing current flow and heat output. If a current is applied to a strip of such material, the aforementioned process occurs automatically and, most importantly, independently at each location along the strip, i.e., the heat output at any location is dependent almost entirely on the temperature of the strip at that location.
The manufacturer of the Raychem heaters indicates that the product is useful in distributing heat across the trailing end of an airplane wing during composite repair; that the product can be used to protect various transit, communication and weapon systems against snow and ice; that the product can be used to defrost mirrors (such as automobile rear-view mirrors); that it can be used as a diesel fuel line gel preventing system; and, that the trademarked THERMOSPACE.TM. convection heaters can be used to warm enclosed areas.
The present inventor is aware of one explored application using the Raychem self-regulating heaters, i.e., an attempt to use such heaters to warm enclosed dough proofing cabinets. Testing of such application did not prove to be useful, and to the knowledge of the present inventor the Raychem products have not heretofore been successfully used in the food manufacturing or food service fields.
To provide an improved food warmer system which overcomes the drawbacks mentioned above would be a significant advance in the food service field, especially if such system also possessed the controlability and power reduction advantages of the aforementioned or other self-regulating warming devices.